During a total hip arthroplasty, a femoral stem is implanted into the intramedullary canal of a femur. After the stem is inserted to the proper depth and orientation, a femoral head or ball is attached to the proximal end of the stem. This head fits into the socket of an acetabular component and provides a joint motion surface for articulation between the femoral prosthesis and acetabulum. A neck or trunion extends between the femoral ball and stem. In many embodiments, this neck generally has a cylindrical configuration with one end connected to the ball and one end connected to the stem.
Several critical features are important to ensure that the femoral hip prosthesis properly functions once implanted in the femur. One of these features is the femoral head “offset.” Femoral offset is the horizontal distance from the center of rotation of the femoral head to a line bisecting the long axis of the femur from a standing A-P x-ray. Similarly, the offset of the proximal femoral component of a hip prosthesis is the horizontal distance from the center of rotation of the femoral head to the long axis of the stem.
One important decision that must be made during hip surgery is how much offset should occur between the femoral ball and stem. If the offset does not match the natural anatomical needs of the patient, then the prosthesis can be positioned too far laterally or medially. An unnecessary decrease in offset greatly affects the success and proper function of the hip implant after surgery.
A decrease in femoral offset medially shifts or moves the femur closer to the pelvis. This decrease can result in impingement of the prosthesis for some patients after surgery. A medial shift can also cause soft tissue surrounding the implant to become loose or lax. Impingement and soft tissue laxity can further lead to instability of the implant, subluxation, and even dislocation. As a further disadvantage, when the offset decreases, the abductor muscles utilize a greater force to balance the pelvis. This increase in force creates a discrepancy that may result in a limp for the patient. As another consequence, the resultant force across the hip joint also increases, and this increase can lead to greater polyethylene wear between the femoral ball and prosthetic acetabular component.
In contrast to a decreased offset, an increase in femoral offset laterally shifts or moves the femur farther from the pelvis. In some instances, an increase in offset is desirable. This increase can reduce the risk of impingement and improve soft tissue tension, resulting in a more stable implant. Further, the adductor muscles can be more properly balanced and improve the gait of the patient. Further, proper balance and alignment can lead to less wear and loosening over time.
Manufacturers and designers of femoral hip prosthesis recognize the shortcomings associated with decreased offset and endeavor to match the offset with the anatomical needs of the patient. In order to remedy these shortcomings, femoral hip prostheses are sold with different offsets. The number and degree of different offsets vary between the manufacturers. A typical prosthetic system can include three to five different offsets for each femoral ball size. For example, a manufacturer may provide femoral balls with offsets of −4 mm, 0 mm, +4 mm, +8 mm, and +12 mm. These offsets would be available for five or six different ball sizes. In short, the manufacturer is required to have an inventory of 18 to 30 different femoral heads.
An inventory of femoral heads of this magnitude is enormous. Further, the costs associated with maintaining and distributing this inventory are very great for a company. This large inventory, then, is a clear disadvantage.
As another important disadvantage, manufacturers offer the femoral head offsets in fixed, discrete, large increments. As noted, the offsets, for example, may be offered in increments of 4 mm, such as offsets of −4 mm, 0 mm, +4 mm, +8 mm, and +12 mm. These fixed increments though may not exactly match the anatomical offset that the patient needs. For example, if the patient requires an offset of +6 mm, then the surgeon must choose between an offset of either +4 mm or +8 mm.
It therefore would be advantageous to provide a proximal femoral head having a variable offset that is selectively adjustable to conform to various anatomical conditions encountered during a femoral surgical procedure.